This project is aimed at determining whether vasopressin (AVP) is involved in human hypertension by 3 types of experiments: 1) Arginine vasopressin (AVP) is given to normal human volunteers to determine whether blood pressure will rise either by an immediate vasoconstrictive action or by promoting fluid retention and hypervolemia. This will be ascertained by observing the responses in blood pressure, fluid balance, plasma volume and extracellular fluid volume, to the administration of AVP by acute intravenous infusion and by chronic (2 week) subcutaneous injections of Pitressin tannate in oil, to normotensive volunteers. 2) Plasma vasopressin concentrations will be measured in hypertensive patients who have been classified according to the pathogenic mechanism of their hypertension into low renin hypertension, angiotensin-dependent hypertension, primary aldosteronism, renoprival hypertension, normal renin essential hypertension, malignant hypertension and other specific types of hypertension. Plasma AVP will be measured before and during observation of the excretory responses to a water load both recumbent and upright. 3) Effects on blood pressure will be studied in the same subjects, of the induction of loss of body water by diuresis and Na replacement and by inhibition of AVP action by administration of demeclocycline. Improvements in understanding the pathogenesis of some types of hypertension, and its treatment should result from the study.